1. Field of Invention
The invention relates to integrated circuit (IC) metrology and more particularly to the use of metrology systems and profile libraries or profile data spaces to determine the critical dimensions (CD""s) and profiles of IC structures.
2. Related Art
With the current drive towards smaller geometries of IC features, features measurement is increasingly difficult as the size of the features become smaller. However, knowledge of the dimensions of gratings or periodic structures is essential in order to determine if the dimensions of the features are within the acceptable ranges and if a particular fabrication process causes the sidewalls of the features to be tapered, vertical, T-topped, undercut, or have footings.
Traditionally, a sample was cleaved and examined with a scanning electron microscope (SEM) or similar device. The cross-section SEM method is typically slow, expensive, and destructive whereas the CD SEM method only provides one measurement number seen from the top of the feature. Spectroscopic reflectometry and ellipsometry are used to beam light on the structure and measure the spectrum of reflected signals. Current practices basically use an empirical approach where the spectra of reflected light is measured for a known width of features in a structure. This process is time consuming and expensive even for a limited library of profiles of structure dimensions and the associated spectrum data of reflected/diffracted light. Furthermore, if such a library were built for a wide range of profiles at a fine resolution, the process of building the library is time consuming and cost-prohibitive. With IC features becoming smaller, there is a need for a library with even finer resolution. As the resolution of the library increases, the size of the library increases while the time to create the library increases exponentially. Furthermore, an extensive library of profiles and spectra is inefficient for searching purposes, especially for real-time work. Thus, there is a need for a method, system, and/or apparatus that facilitates the use of metrology devices and systems for measuring profiles without creating huge profile libraries or data collections and incurring extensive searches of said profile libraries or data collections.
The present invention includes a method and system for determining the profile of an integrated circuit structure from a measured signal by selecting one or more best matches of the measured signal in a profile data space and performing a refinement procedure to determine refined profile parameters. The measured signal may be generated by an optical, electric, electron, or mechanical metrology device. The profile data space may be a profile library of profile parameters and corresponding signals or a collection of data points representing profile parameters and corresponding signals.
An exemplary embodiment includes a method and system for ensuring that a specified extent of non-linearity between data points exists so as to ensure consistent results from the refinement calculations.
Another exemplary embodiment includes a refinement procedure comprising finding a polyhedron in the data space of profile parameters, the polyhedron configured to contain the profile parameters of the best match library spectrum and also configured such that the corners of the polyhedron correspond to selected profile parameter data point, one for each profile parameter. The total cost function of the best match spectrum and spectra of the selected data points compared to the measured spectrum is minimized utilizing a weighting factor. An alternative approach is finding a polyhedron configured to contain the profile parameters of the best match library spectrum and also configured such that the corners of the polyhedron correspond to selected profile parameter data point, two for each profile parameter. Still another embodiment includes calculation of the refined profile parameters by using a sensitivity matrix to determine profile parameter adjustment values.
Alternatively, a clustering approach is used to select cluster representatives for each cluster of profile library instances and to derive an adjustment multiplier for each profile parameter value of the cluster representative. The refined profile parameters are calculated by multiplying the measured diffracted spectrum by the corresponding adjustment multiplier for each profile parameter value of the best matching cluster representative. In another application, the adjustment multiplier is derived from selected profile library instances according to selection criteria.
Yet another embodiment is a regression-based refinement method wherein data points within a data space of signals and profile parameters are successively evaluated for goodness of fit compared to the measured spectrum. Subsequent regression data points are selected using global and local optimization techniques. The signals for subsequent regression data points are calculated using metrology simulation procedures.
Furthermore, the present invention may utilize localized fine-resolution refinement library methods, iterative library refinement methods, other cost optimization methods, and/or other refinement algorithms or techniques. The refinement procedure may be invoked automatically or invoked based on occurrence of predetermined criteria such as exceeding an error metric between the measured signal versus the best match profile library signal.